1. Field of the Invention
This invention relates to an improved device for adjusting a developing solution for an electrostatic wet-type electrophotographic printer.
2. Description of the Related Art
Electrostatic wet-type electrophotographic printers have been known already, for example, in Japanese Unexamined Patent Publications No. H7-44025 and H11-282256, namely 44025/1995 and 282256/1999. In each printer, the toner concentration and volume of a developing solution in a storage tank are maintained in predetermined ranges by controlling the amounts of a liquid carrier and of a concentrated toner fed into the storage tank.
The electrostatic wet-type electrophotographic printers of this kind are equipped with a concentration sensor for measuring the toner concentration of the developing solution in the storage tank and a level sensor for measuring the volume of the developing solution in the storage tank. In addition, control means executes the arithmetic operation based on the values detected by the sensors and controls a carrier feed pump and a toner feed pump in order to maintain the toner concentration and volume of the developing solution in the storage tank within predetermined ranges.
That is, when the toner concentration of the developing solution is unnecessarily high, the carrier feed pump is driven to feed the liquid carrier into the storage tank thereby to decrease the toner concentration. When the toner concentration is low, on the other hand, the toner feed pump is driven to feed the concentrated toner into the storage tank to increase the toner concentration.
Further, when the volume of the developing solution becomes unnecessarily large in the storage tank, a developing solution drain pump is driven to drain an excess of the developing solution. On the other hand, when the volume of the developing solution becomes insufficient, the carrier feed pump is driven to feed the liquid carrier into the storage tank and thereby to increase the amount of the developing solution.
According to the conventional electrostatic wet-type electrophotographic printers disclosed in Japanese Unexamined Patent Publications No. 44025/1995 and 282256/1999, however, the carrier feed pump and the toner feed pump are driven only during a time interval in which the printing operation is not executed. When the printing operation is continued for extended periods of time, therefore, the toner concentration and volume of the developing solution fluctuate to undesired levels, which may make it difficult to properly conduct the printing. This is because the carrier feed pump and the toner feed pump do not operate so far as the printing operation is conducted.
When the volume of the developing solution becomes insufficient, the carrier feed pump only is driven to feed the liquid carrier. This causes such a trouble to occur such that the toner concentration of the developing solution decreases temporarily.
Further, the passage has been opened at all times between the storage tank that stores the developing solution and a developer, and the developing solution feedback passage has also been opened at all times to feed the developing solution used by the developer back to the storage tank. When the device is left to stand for extended periods of time, therefore, the liquid carrier volatilizes through these portions; i.e., the concentration of the developing solution becomes dense so that the printing operation is impaired when it is subsequently executed.
It is therefore an object of this invention to provide a device for adjusting a developing solution for an electrostatic wet-type electrophotographic printer, which is free from the defects inherent in the above prior art.
It is a specific object of this invention to provide a device of the type described, which is capable of flawlessly executing the printing operation continuously for extended periods of time while properly maintaining the toner concentration and volume of the developing solution at all times. Such a device does not permit the liquid carrier to be undesirably volatilized even when the device is left to stand.
This invention is concerned with a device for adjusting a developing solution for an electrostatic wet-type electrophotographic printer. Specifically, the device comprises: a storage tank for storing the developing solution; a carrier cartridge for storing a liquid carrier; a toner cartridge for storing a concentrated liquid toner; a carrier feed pump for feeding the liquid carrier in the carrier cartridge into the storage tank; a toner feed pump for feeding the concentrated toner in the toner cartridge into the storage tank; a storage tank stirrer for stirring the liquid carrier and the concentrated toner fed into the storage tank to prepare a developing solution; a developer for effecting the developing with the developing solution fed from the storage tank; a developing solution feedback passage for feeding the developing solution used by the developer back to the storage tank; and developing solution drain means for draining an excess of the developing solution in the storage tank into the toner cartridge.
In order to accomplish the above object, provision is further made of: a concentration sensor for measuring the toner concentration of the developing solution in the storage tank; a level sensor for measuring the volume of the developing solution in the storage tank; and control means for controlling, in real time, the carrier feed pump and the toner feed pump based upon values detected by the concentration sensor and the level sensor during the printing operation, in order to adjust the volume and the toner concentration of the developing solution in the storage tank so as to lie within predetermined ranges.
The toner concentration and the volume of the developing solution in the storage tank are measured during the printing operation, and the carrier feed pump and the toner feed pump are controlled in real time based on the measured values. Accordingly, the toner concentration and the volume of the developing solution are adjusted even during the printing operation. Even when the printing operation is continued for extended periods of time, therefore, the toner concentration and the volume of the developing solution are maintained within suitable ranges, and the printing operation is properly conducted at all times.
Further, the developing solution drain means can be constituted by a developing solution drain pump. In this case, the control means is furnished with a function for controlling the developing solution drain pump to control the developing solution drain pump in addition to controlling the carrier feed pump and the toner feed pump.
It is also allowable to constitute developing solution drain means by utilizing the overflow from the storage tank. In this case, however, if an overflow takes place as a result of feeding the concentrated toner or the liquid carrier for adjusting the toner concentration of the developing solution, it is probable that the unadjusted developing solution only existing in the storage tank may overflow, or the newly fed concentrated toner or the liquid carrier only before being stirred may overflow, without properly adjusting the concentration. The developing solution drain pump is driven to maintain the level of the developing solution within a predetermined range in the storage tank. This makes it possible to prevent undesired overflow, and to properly adjust the concentration upon feeding the concentrated toner and the liquid carrier and, particularly, to adjust the concentration during the printing operation.
It is further desired that the control means is provided with a function for feeding the developing solution of a standard concentration by driving the carrier feed pump and the toner feed pump for only a preset period of time in feeding the developing solution.
Then, upon properly setting a ratio of operation times of the carrier feed pump and the toner feed pump, the developing solution of a standard concentration can be fed into the storage tank. This eliminates a defect in the prior art in that the toner concentration fluctuates as a result of feeding the liquid carrier only.
Moreover, the storage tank is of a sealed structure, normally-closed valves are disposed between the developer and the storage tank and in the developing solution feedback passage, and the control means is provided with a valve opening/closing function for opening or closing the valves upon detecting the start and end of the printing operation.
According to this constitution, the storage tank is completely sealed at all times when the printing operation is not being executed, preventing a change in the concentration of the developing solution caused by the volatilization of the liquid carrier.
When the normally-closed valves are disposed between the developer and the storage tank and in the developing solution feedback passage, it is desired that the control means is provided with a valve closure delay function for inhibiting the closure of the valves at the time of adjusting the volume and the toner concentration of the developing solution in the storage tank.
According to this constitution, the developing solution is maintained circulating between the storage tank and the developer even after the end of the printing operation until the completion of adjustment of the volume and toner concentration of the developing solution. Even in case the adjustment of the volume and the toner concentration of the developing solution is started just at the time of ending the printing operation, therefore, the unadjusted developing solution does not remain in the flow passage connecting the storage tank to the developer, and the developer commences the processing with the developing solution of an optimum concentration from a moment when the printing operation is started next.
In the case of an electrostatic wet-type electrophotographic color printer, it is necessary to provide storage tanks, storage tank stirrers, toner cartridges, toner feed pumps, developing solution feed pumps, developing solution feedback passages, concentration sensors and level sensors in a plural number to meet the number of the developers, but the carrier cartridge and the carrier feed pump may be provided in a number of one, respectively.
In this case, the storage tanks and the carrier feed pumps are connected together through the normally-closed carrier feed valves provided for the storage tanks, and the control means is provided with a selection function for selecting a toner feed pump, a developing solution drain pump, a concentration sensor, and a level sensor and a carrier feed valve to be opened, that are corresponding to a storage tank in which the volume and the toner concentration are to be adjusted.